The GAPN tests are well worth it if you are unsure of your aptitude. They have been developed over the past 50 years, originally from RAF selection (and are still used..)
It's not a, 'who knows the most maths' test (as you can learn all that), but more an inate abilities test. Either you have good co-ordination or not. You can either multi-task efficiently or not, either you are vigilant or not, either you can see shapes relating to instruments or not.
I took it and received a score that set my mind at rest. To back this up, Cabair, Oxford (in their soon to launch APP program) and BAe all use modified versions of it and they all informed that if I had the GAPN test under my belt with a good score, then I would not need to complete their version as it would suffice for that particuliar entry purpose.
I'm not sure if there is anything you can do to 'prepare'... that wouldn't really be the point of the test. You just want to turn up and get a good score. It will mean far more about your inate abilities that way as you have never done it before. You will not be bluffing your abilities when push comes to shove later on.
Personally, I just went along and did it from scratch.
Cheers,
Craggenmore